I dont understand what does the automatic or manual change over thermostat mounted on the FCU ! What does he change over ? On winter on the innlet I have hot water and on the summer I have cold water !
If it's a two-pipe change over system, then there is only one coil in the FCU. In the summer, it gets supplied with chilled water from the central plant, in winter it gets supplied with hot water from the central plant.
If this is your setup, then you need a summer/winter change over switch so that the FCU controller knows whether to open the valve as temperature rises (summer) or close the valve as temperature rises (winter).